Light modulation system



ct. 12, 193?. H. SHORE ET AL LIGHT MODULATION SYSTEM Filed May 26, 1934 7'0 f/E/ITf/PS //V PARALLEL A AAAQAAAA lllAAl vvvvv 56' (ARR/El? FREQUENCY lA/PUT INVENTOR f/fA/Af 67/0/75 JAM-s L/fi ATTORNEY Patented Oct. 12, 1937 r UNITED STATES- PATENT ferries Whitaker, TuckahoeyN. Y., .assignors to Radio Corporation of America, a corporation of Delaware Application May 26,1934; Serial no. 727385 6 Claims. (01. 179- 171) The present invention relates to picture transmission systems and finds application principally in connection with facsimile and television transmission of pictures and other similar and related subjects. a

The invention also finds application in other arts, such, for example, as color sorting, smoke detection, meter reading, light control apparatus, electric sign control, and other associated and allied arts.

For the purpose of illustration, however, the invention has herein been applied to the picture transmission art in order to illustrate one of the many general applications thereof.

As a primary object of the invention, we have sought, to provide a system in which the fidelity of picture transmission and the like will be improved and wherein the resulting output will be linear as a function of the density of the subject of which an electro-optical reproduction is desired.

due to its extreme stability, will hold in adjustment without change for long periods of time so that especially'in connection with the transmission of pictures re-runs may be made'without the loss of time for readjustment and thereby save considerable circuit time.

As a further object of the invention, we have sought to provide a light modulating system wherein great simplicity of operation and manipulation for optimum conditions of operation is inherent.

Further" objects and advantages of the invention will be pointed out in connection with the description of the apparatus illustrated and de scribed herein and still other objects and advantages will naturally suggest themselves to those skilled in the art to which the invention is directed by reading the following descriptionand claims in connection with the accompanying drawing wherein the single figure thereof illustrates schematically one of the many forms which our invention may assume.

Referringnow to the drawing, and bearing in mindthat'the invention herein described is illustrated in connection with only one of its many forms of application, where it is desired to trans- As a further object of the invention, we have sought to provide alight conversion system which mit pictures, there is provided a record carrying drum l bearing the record strip or picture 3. Preferably the drum is rotated by any desired and suitable form of drive such, for example, as has'been illustrated in United States Patent No. 1,803,133 grantedtto H. Ranger on April 28, 1931. Whenthe facsimile drive is set up'as above suggested, light from any suitable and desired source 5 is focused 'by means of an optical system I at a. point 9 on the record surface' The reflectedlight of thepicture subject at the point 9 is then directed by means'of a second optical system I l to influence the light-sensitive electrode of a light converting'meansconventionally indicated by the photoelectric cell I3. The photoelectric cell l3 has one of its'electrodes, preferably the light-sensitive cathodethereof, connected to influence the parallellyconnected grid or control electrodes of the vacuum tube i5. The tube l5; as are all'oth'ertubesdisclosed' herein, is preferably ofthetype'known inthe art as the"!!! tube which is of'the general type combining within one bulb two high-mu triodes. The other electrode of the photoelectric cell [3 is arranged to connect with the parallelly connected grid or control electrodes of a'second tube I! of the same general type as the tube l5.

Voltage is supplied to the electrodes of the photocell preferably from-a common voltage divider'so that the cathode of' the photocell connects, for example, to a negative point IS on the voltage divider by way of the secondary winding 2| of the transformer 22 and the resistor 23, whereas thepositive electrode of the photo cell l3 connects 'to'a point 25 more positive than the point I9 .by way of a second secondary winding 21 of the transformer 22 and the resistor 29.

The grid electrodes of the tubes I5 and 11 are also supplied with alternating current of a predetermined carrier or tone frequency which is connected to the terminal points 3| so as to flow through'the primary windings 33 of the. transformer 22. The secondary windings 2| and 2! of this transformer are preferably reversed with respect to each other so that, for example,

, cathode electrode of the tube I5 is connected to end of the voltage divider 3| a point 51 on the voltage divider, whereas the cathode of the tube l! is connected to a point 59 on the voltage divider 3|. "L

The output current from the tube which is connected push-pull fashion flows through'the primary winding 6| of the transformer 63 so as to induce in the secondary winding'65 thereof suitable voltages which can be measured by means of a meter 61 connected *across the sec-' ondary winding. At the output terminals fit connection may be made to a suitable type of load circuit in the form of a rec'orderor'to a transmitter of either the radio or the wire type; Plate voltage for'the tube 35 is supplied by a connection at point II to the common voltage divider 3|. The point H is maintained asthe highest point in the system and the opposite is preferably grounded at a point 13.

In'order to supply heating current for all of the tubes I5, I! and 35, a separate heater circuit conventionally indicated as 15 is arrangedto supply all heater elements in parallel and is preferably energized from a suitable source which has been conventionally indicated as an 8-volt source.

When a device of the type herein disclosed is operating, it will be seen that the system disclosed is that of a balanced grid modulator whichis followed by a single stage of push-pull amplifi- 1 cation. In the balanced grid modulator comprising the tubes I5 and I! the tone to be modulated which is applied at the terminal points 3| is fed to the grids of the modulator tubes in like phase. These tubes l5 and H are arranged to have essentially the same bias and this bias is purposely made high so as to operate inthe curved region of the grid voltage-plate current characteristic. If the tubes l5 and H are identical the carrier tone causes an equal rise and fall of potential in the plate circuit of each tube. The fact that the plates are coupled together in anti-phase relation causes the two potentials to buck out each other so that the output current is normally zero. The modulating signal supplied by way of the light modulation influencing the photoelectric cell l3 actuates the grids of the tubes 15 and IT in push-pull and serves to decrease or lower one grid potential while the potential of the other grid is raised, and since the tubes l5 and'll are assumed to be operating in the curved region of the characteristic, changes in grid potential cause the amplification factor of the tube to vary, increasing negative potential, for example, causing a decrease of gain and vice versa. Consequently, the modulating voltage by raising the amplication in one tube produces an unbalance of voltages in the plate circuit. It follows, then, that since the two voltages are no longer equal a voltage equal to the diiference betweenathe two plate voltages is now available as the output;

This difference in voltage is proportional to. the shift in grid potential over a relatively large range and accounts for linearity of output as a function of input. 1

site, phase and so perform the duty of modula- "tion. In order to raise the output to any desired degree the modulated carrier tone may then, as shown, be amplified by a single or multiple pushypull stage and it will be appreciated that this 1 arrangementfurnishes a means by which the output may be kept free from harmonics and furnishes a wave across the terminal point 59 i which is generally'of a pure sine wave shape.

While We have illustrated by the accompanying drawing only one suitable form which our in- 1 vention may assume, it is, of course,-to be understood that the said invention may assume and take many varied forms without departing from thespirit and scope of this disclosure and we therefore believe ourselves to be entitled to make and use any and all of these modifications which would iatonce suggest themselves to those skilled in the art insofar as such modifications and changes fall fairly within the invention as defined by the hereinafter appended claims.

Having thus described our invention, what we claim and desire to secure by Letters Patent is the following:

l. A light controlled modulation system comprising a plurality of pairs of thermionic devices each having control grid and plate circuits parallelly connected, a photoelectric cell having its light sensitive cathode connected to one pair of parallelly connected control grids and its anode" connected ,to the other pair of parallelly connected control grids, a source of carrier frequency, a plurality'of circuits each including independent resistance and inductance elements for supplying voltages to the photoelectric cell, said inductance elements also serving as reversely wound secondary transformer windings, means including said transformer windings for introducing A. C. voltages of instantaneous like sign upon the electrodes of said photoelectric cell and each pair of parallelly connected control grid elements so that said A. C. is 'modulated by voltage changes on the control grids due to variations in light on the photocell and so that stray capacity of the photoelectric cell to ground is reduced to a minimum, and a load circuit for amplifying the produced modulations.

2. A modulation system comprising a unidirectional signal energy source, a pair of thermionic devices having control electrodes, input circuits and output circuits, said thermionic devices having their input circuits connected in opposite phase with the said source, a source of carrier frequency connected in like phase with said thermionic devices, a balanced impedance input means for supplying carrier frequency to the said thermionic devices connected in opposite phase so as to reduce the stray capacity to ground to a minimum value, a common voltage supply for all of said devices, means for supplying independently variable biasing voltages to the control electrodes of the thermionic devices from the common voltage supply, and a load circuit connected with the output circuit of said thermionic devices.

3. Themodulation system claimed in claim 2 wherein said load circuit comprises a pair of push-pull connected thermionic devices connected with the output circuit of said first-named thermionic devices.

4. In a modulation system, a light responsive electrical device, a plurality of series connected impedance elements connecting between the anode and cathode of the light responsive device, parallelly connected thermionic devices having control electrodes, input circuits, and output circuits, said parallelly connected thermionic devices having their input'circuits connected with the two end portions of the series connected impedance elements so that the light responsive device varies the output energy from each parallelly connected thermionic device in dependence to the intensity of light impinging upon the light responsive elements to vary the voltage drop through the said series connected impedance, a single power supply for energizing the thermionic devices, means to supply independentlyvariable biasing voltages to the parallelly connected control electrodes from the single power supply, means for impressing upon each of the parallelly connected thermionic devices a carrier frequency to produce like changes in voltage on the control electrode of each, a balanced impedance means for supplying said carrier to each of said parallelly connected thermionic devices to reduce stray capacity to ground to a minimum value, means to determine the change in output energy from each pair of parallelly connected thermionic devices, and a load circuit connected with the output of said first thermionic devices.

5. In a modulation system, a light responsive electrical device, a plurality of impedance elements comprising series connected resistances and transformer secondaries connected across the light responsive device output, parallelly connected thermionic devices having control electrodes, input and output circuits, said thermionic devices being coupled to the electrode of the light responsive device and the end terminal points of the series connected impedance elements so that the input of each parallelly connected thermionic device rises and falls in dependence to its connection in accordance with the light actuating the light responsive element, a single power supply for energizing the thermionic devices, means to supply independently variable biasing voltages to the parallelly connected control electrodes'from' the single power supply, means for impressing upon all of the parallelly connected thermionic devices a carrier frequency to produce like changes in voltage on the control electrode of each, a balanced impedance meansfor supplying said carrier to each of saidthermionic devices to reduce stray capacities to ground to a minimum, and a load circuit including a pair of pushpull connected thermionic devices connecting with the parallelly connected output of said firstnamedtliermionic devices.

6. A modulation system comprising a uni-directional signal energy source, a pair of thermionic devices having control electrodes, input circuits and output circuits, said devices having their input circuits connected push-pull fashion with the uni-directional signal energy source, a source of carrier frequency connected in like phase with each of said thermionic devices, and a balanced impedance input means for supplying the carrier to the said push-pull'connected thermionic devices, a single power supply for energizing the thermionic devices, means to supply independently variable biasing voltages to the control electrodes of the thermionic devices from the single voltage supply whereby output energy from the thermionic devices either increases or decreases in accordance with increasing output energy from the uni-directional signal energy 

